1. Field of the Invention
The invention relates to an apparatus, a method, and a computer program product for deconvolution analysis used for calculation of true depth profile from depth profile resulting from a depth analysis by using the sputtering surface analysis such as secondary ion mass spectrometry (SIMS), x-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) on a sample to be evaluated including a sheet metal and a semiconductor material.
2. Description of the Related Art
As a method of measuring the depth concentration profile of impurities within semiconductor material such as a sheet metal and a complementary metal oxide semiconductor (CMOS), there have been. known SIMS, XPS, and AES, which are depth analysis methods for the composition of a thin film surface. These methods of depth analysis irradiate the surface of a sample with ion beams from some 100 eV to some 10 keV and sputter atoms from the sample surface, hence to do depth analysis of the elements present in the vicinity of the sample surface.
These depth analysis methods of the composition on the sample surface, however, have such a problem that they cannot correctly estimate depth profile of a very thin film and a sharp dept profile of impurities because the atoms on the sample surface are mixed together in some deep region through irradiation of the energized ion beams (mixing effect). Here, the lower energy of irradiating ion beams may be used, but in the case of measuring an extremely thin film (for example, a thin film of 5 nm or less) or a sharp impurity profile (for example, depth profile of the impurity elements ion-implanted with an energy of 1 keV or less), it is impossible even by use of ion beams of the lower energy (1 keV or less), to obtain the true depth concentration profile owing to the mixing effect caused by the primary ions within the sample. Although the energy of the ion beam may be further lowered, a lower limit for the energy may be set to 200 eV, taking the performance of an ion gun into consideration in the present circumstances. Even when an ion gun which can emit ion beams of 100 eV or less is developed in the future, there occurs little ion sputtering in ion beams of 100 eV or less, hence to make it impossible to do analysis.
Owing to this restriction on the analysis, in order to obtain a true, or an approximately true depth profile, there arises a necessity of using a deconvolution analysis method for estimating the true depth profile from the measured depth profile, considering the depth resolution concerned with the phenomenon caused by the ion sputtering, such as the mixing effect. Specifically, assuming that a measured profile is defined as I(z), a depth resolution function, a function representing the distortion caused by analysis, is defined as g(z), and that a true depth profile is defined as X(z), these three functions can be expressed in equation (1) (See FIG. 12):I(z)=∫X(z′)g(z−z′)dz′  (1)
In short, the deconvolution analysis method is a method for estimating a true depth profile X(z) from I(z) considering this depth resolution function g(z).
In the phenomenon occurring in the initial sputtering in the vicinity of the sample surface, when the sample surface is irradiated with any other elements than rare gas, since the atomic species irradiated as ion remains on the sample surface, its concentration gradually increases until it becomes constant (it reaches the steady state). In other words, in the initial sputtering, the concentration of the atomic species of the ion irradiated on the sample surface and the atomic species forming the sample surface varies gradually. Even when rare gas is used as the irradiating ionic species, when the atomic species forming the sample surface is two types and more (for example, SiO2, NiSi, GaAs, and so on), a phenomenon called selective sputtering occurs, although it depends on the irradiation conditions such as energy and incident angle, and the concentration of the atomic species of the irradiated ion on the sample surface and the atomic species forming the sample surface will gradually change until it reaches the steady state.
When the concentration of the atomic species forming the sample surface changes, the depth of sputtering per time unit, namely the sputtering rate also changes simultaneously. That is, the measured depth profile becomes different from the actual depth profile, owing to a change in the sputtering rate. Therefore, in the case of deconvolution analysis on the depth profile of concentrated boron present in the vicinity of the surface, the depth profile distorted by a change of the sputtering rate has to be returned to the accurate profile and analyzed; otherwise, a wrong profile would be obtained.
According to C. F. McConville, S. H. Al-Harthi, M. G. Dowsett, F. S. Gard, T. J. Ormsby, B. Guzman, T. C. Q. Noakes, P. Bailey, J. Vac. Sci. Technol. B20 (2002) 1690 (hereinafter, “C. F. McConville et al.”), in the case of sputtering a Si sample with oxygen ion beams, medium energy ion scattering (MEIS) measures that in the initial sputtering, the sputtering rate changes in the dept direction (the sputtering rate is greatest on the uppermost surface and then gradually decreases, to a constant value). More specifically, a sample whose surface was amorphized with about 15 nm is irradiated with ion and at once measured in the MEIS, thereby obtaining the decreasing amount in the width of the amorphous region, in the other words, the relationship between the sputtered depth and the ion irradiation dose.
L. Shao, J. Liu, C. Wang, K. B. Ma, J. Zhang, J. Chen, D. Tang, S. Patel, W. K. Chu, Appl. Phys. Lett. 83(2003) 5467 (hereinafter, “L. Shao et al.”) discloses a technique of converting an apparent depth profile distorted by a change of the sputtering rate into a true depth profile by calibrating the depth change of the sputtering rate in a depth direction in the initial sputtering. According to this technique, the apparent depth when it is calibrated assuming that the sputtering rate is constant in the depth direction, is corrected by using equation (2):Zreal=Zapp+a×(1−exp(−b×Zapp))  (2)where Zreal indicates the axis of an actual depth (nm), Zapp indicates the axis of an apparent depth (nm) calibrated. assuming that the sputtering rate is constant in the depth direction, and a and b indicates coefficients (nm, nm−1).
In the case of using SIMS, since the signal intensity (secondary ion intensity Y) obtained depending on a change in the sputtering rate also changes, it is necessary to calibrate the signal intensity in equation (3):Yreal=Yapp/(1+a×b×exp(−b×Zapp))  (3)
In AES and XPS, calibration by equation (3) is not necessary.
The parameters a and b of equation (2) may be derived from the result of MEIS measurement like C. F. McConville et al., or they may be estimated from a comparison of the measurement profile between an ion-implanted sample and a capped sample with a film some nm to some 10 nm thick of the same material as the substrate. In the latter method, the sputtering rate becomes constant in the ion-implanted region by depositing the film of the same material there, a comparison between this depth profile and the depth profile for the ion-implanted sample (sample without cap film) has only to be made, so to estimate the parameters a and b.
However, after the concentrated boron profile (formed by the ion implantation) present in the vicinity of the surface is measured by the SIMS, when the obtained data is calibrated with the sputtering rate in the method of L. Shao et al. and analyzed with deconvolution, a depth profile indicating a physically impossible structure (a concentration of zero in the vicinity of the surface and the vibration structure in the vicinity of the depth 2 to 3 nm) is derived in the initial sputtering disadvantageously. L. Shao et. al. discloses that this peak of about 10 at % concentration is significantly larger. This peak would be considered an artifact on the deconvolution analysis. This is because of the following reasons.
It can be easily understood that in the region where the concentration of the atomic species forming the sample surface gradually changes in the initial sputtering and the sputtering rate also changes, the depth resolution function also changes in parallel. When such an analysis condition is used that sputtering causes no surface roughness, the depth resolution function is considered to be constant at a certa depth and more. Although a test of examining a change in the depth resolution from the uppermost surface to a point when it becomes constant has not been reported, the change is assumed as follows. In the right initial sputtering (when one ion beam first bumps against the sample), naturally no mixing occurs in the sample and so the depth resolution function is a delta function. When the sputtering comes into a constant state and the sputtering rate becomes constant, since the depth resolution function is in a shape of having some width, it is naturally considered that the depth resolution function is changing between a period from the right initial sputtering to the time when the sputtering rate becomes constant.
Namely, when the depth profile of the concentrated boron present in the vicinity of the surface is analyzed with deconvolution, since the concentrated boron is distributed in the region where the depth resolution varies, a change in the depth resolution has to be taken into consideration together with a change in the sputtering rate. Otherwise, a wrong depth profile would be derived.